What causes the ionosphere to form?

The ionosphere is a region of the upper atmosphere heavily populated by free electrons and positive ions. It is formed when intense solar radiation from the sun strikes the thin gas molecules in the outer atmosphere, stripping electrons away from their atoms in a process called ionization. Distractors like lightning and temperature changes are incorrect because they occur in the troposphere, the lowest layer of the atmosphere where everyday weather happens. The immense energy required to maintain the vast ionized layers necessary for radio propagation comes almost entirely from the sun.